In the chemical industry a common technique for purifying or analysing chemicals is an exchange process. Solvent extraction relies upon the preferential transfer of one or more components from one phase (fluid) in which the component (solute) is dissolved into a second immiscible phase. Usually this is accomplished by physical mixing followed by separation of the two phases using gravity. It has been found that the more thoroughly the two phases are mixed, the more rapidly the transfer process proceeds by reason of the greater surface area of the smaller globules of liquid and reduced diffusion distances within the phases. The time for separation of the phases however increases with more thorough mixing, and hence for a desired efficiency of solute transfer, the separation time may become unacceptably long, this being the principal disadvantage of the process.
Our co-pending International Application PCT/GB95/02489 discloses a method and means of bringing first and second immiscible fluids in contact with one another for interaction, while inhibiting physical mixing of the fluids, to permit easy separation of the fluids subsequent to interaction, and claims apparatus for carrying out a process between first and second immiscible fluids. The apparatus comprises first and second flow paths for permitting fluid flow of respective first and second immiscible fluids therethrough. Portions of the flow paths are disposed close to, or adjacent to, one another and communicate with one another in a region which is such as to permit the fluids to form a stable open interface therein. At least the first flow path in the interface region has a width normal to the interface within the range 10 to 500 micrometres.
The International Application also discloses a method of carrying out a process between first and second immiscible fluids, the method comprising:
1) providing first and second flow paths having portions disposed adjacent to or close to one another and communicating with one another in a region in which the fluids can contact one another; PA1 2) flowing the first and second immiscible fluids through respective said first and second flow paths such that, at least in said region, the flow of both fluids is essentially laminar, and a stable open interface is formed between the fluids; PA1 3) permitting significant transfer of a desired component (such as a solute) to another immiscible fluid (such as a solvent) at said interface between the fluids by diffusive transport within the fluids; and PA1 4) flowing the fluids away from the interface region in their respective flow paths without mixing of the fluids. PA1 where .DELTA.P is the pressure drop along a unit length, L, of the flow paths, .mu. is the viscosity of the fluid, l, is the width of the flow path and K is a constant. PA1 a) providing first and second flow paths having portions disposed adjacent to or close to one another and communicating with one another to defme a region where, in use, the fluids can contact one another; PA1 b) flowing the first and second immiscible fluids along respective said first and second primary flow paths such that, at least in said region, the flow of both fluids is essentially laminar and, a stable interface is formed between the fluids; PA1 c) simultaneously causing at least one of the fluids to flow in a direction perpendicular to the primary path flow; PA1 d) permitting significant transfer of a desired component of at least one of the fluids to the other fluid at said interface between the fluids by diffusive transport within the fluids without mixing of the fluids; and PA1 e) flowing the fluids away from the interface region in their respective flow paths.
At the interface region, the flow paths are close to, or adjacent to, one another so that fluid flow through the flow paths continually replenishes the fluid at the interface. An interface is defmed at which the fluids contact one another under defined conditions so that the interface remains stable despite movement of the fluids. Turbulence should not be present at the interface in an amount sufficient to disrupt the interface. For efficiency of operation, portions of the fluids should remain in contact with one another in the interface region for a short time so that throughput of fluids can be maximised. The continual renewal of the fluids at the interface has the additional advantages that degradative side reactions between the fluids and their dissolved components, such as hydrolysis of extractant chemicals, is reduced, as is also the accumulation of undesirable products at the interface. The fluid portions should desirably remain in contact with one another for a period of the order of between 1 and 100 seconds, or more generally between 0.1 and 100 seconds.
It is an object of the present invention to improve the conditions for diffusive transport to take place and encourage a more efficient transport across the interface between the two fluids.
According to one aspect of the present invention there is provided an apparatus for carrying out a process between first and second immiscible fluids comprising first and second channels defining primary flow paths for permitting fluid flow of the respective first and second fluids therethrough, portions of said channels being disposed close to or adjacent one another and constructed to communicate with one another to define a region, where a stable interface is formed between the fluids, and secondary flow inducing means, which is operable, in use, to induce at least one of the fluids to flow in a secondary direction perpendicular to the direction of the primary flow path at said region.
Preferably the secondary flow inducing means comprises constructing at least one of the channels to extend around a curve at said region. Both of the first and second channels may be curved at said region.
In an alternative embodiment the secondary flow inducing means comprises flow deflectors positioned within at least one of the channels, or both of the channels.
The, or each channel may be a spiral channel defining a spiral primary flow path.
According to one aspect of the present invention there is provided an apparatus for carrying out a process between first and second immiscible fluids, comprising first and second channels defining respectively first and second primary flow paths for permitting fluid flow of the respective first and second immiscible fluids therethrough, portions of the channels being disposed close to or adjacent one another and constructed to communicate with one another to define a region where, in use, a stable interface is formed between the first and second fluids, at least the first channel at said region being curved so that, in use, the fluid flow therein has a curved trajectory in a direction along the primary flow path which generates a secondary circulation of the fluid in a direction perpendicular to the direction of the primary flow path.
In a further aspect of the present invention there is provided an apparatus for carrying out a process between first and second immiscible fluids, comprising a stacked structure of adjacent plates, each adjacent pair of plates defining one or more pairs of first and second channels defining primary flow paths for permitting fluid flow of the respective first and second immiscible fluids therethrough, portions of the channels being disposed close to or adjacent one another and constructed to communicate with one another to define a region where, in use, a stable interface is formed between the first and second fluids, and wherein at least the, or each first channel is curved at said region so that, in use, the fluid flow therein has a curved trajectory in a direction along the primary flow path and generates a secondary circulation of the fluid in a direction perpendicular to the direction of the flow path. Preferably strcture of plates is held together by the application of a compressive force.
Preferably the flow rate Q of each of the fluids are between 6.75 E-07 and 1.35 E-05 kg/s, and the flow rate Q is given by: EQU Q=Kl.sup.4.DELTA.P/.mu.L,
According to a further aspect of the present invention there is provided a method of carrying out a process between first and second immiscible fluids, the method comprising: the steps of:
Preferably the step (c) is achieved by causing the flow of fluid to flow along a primary flow path that extends around a curve. The curve is preferably a spiral curve.
Alternatively the step (c) is achieved by means of angled flow deflectors positioned within the, or each, channel.
A foraminated membrane may be provided at the interface between the fluids. If desired the flow deflectors may be fin mounted on one or both sides of the membrane.
In accordance with the invention, an enhanced diffusive transport between the two fluids is achieved by reason of the circulation of the fluid perpendicular to the direction of flow, since this acts to bring fresh regions of fluid into the interface region; diffusion will take place at increased efficiency when the difference in concentration of the component between the two fluids is maintained high. The precise mechanism by which circulation is achieved will be explained in more detail below.